I. Field of the Invention
The present invention relates to internal combustion engines of spark ignited type and is concerned with reducing the emission by such engines of oxides of nitrogen (NOx) and hydrocarbons (HC) and reducing the fuel consumption.
II. Description of the Prior Art
A known method for reducing NOx emissions and fuel consumption of a gasoline engine is to ensure that a lean fuel/air mixture is employed. This has the effect of diluting the fuel with air and reducing the combustion temperature. Lower combustion temperatures tend to result in reduced NOx emissions. Reduced throttling losses and better combustion efficiency of the so-called lean burn engine result in lower fuel consumption. A major disadvantage of this approach is that excess oxygen is present in the exhaust gas. It is therefore not possible to employ a reducing catalyst if further reductions in NOx emissions are required.
It is also known that a similar effect to that achieved with lean burn can be achieved by the use of exhaust gas recirculation (EGR). If, instead of diluting the combustion gases with excess air, they are diluted with recirculated exhaust gas (REG), reductions in temperature can be achieved without the use of excess oxygen. The combustion process can be substantially stoichiometric and it is possible to use a reducing catalyst in the exhaust system for the conversion of residual NOx. A further benefit of using EGR is a fuel economy improvement due to the reduced throttling losses in the intake air stream. However, the use of EGR is also associated with disadvantages if the exhaust gas is distributed throughout the combustion chamber because it adversely affects the initiation and propagation of the flame front and can result in premature termination of combustion and is thus responsible for an increase in the emission of unburnt hydrocarbons.
It is also known that the problems associated with the use of EGR can be largely overcome by configuring the engine such that a stratified charge is produced in the combustion chamber, that is to say by arranging the air and recirculated exhaust gas inlets such that the air/fuel mixture and recirculated exhaust gas occupy separate portions of the combustion chamber and remain substantially unmixed. Additionally, stratification of the exhaust gas over crevices, oil and deposits in the combustion chamber will reduce HC emissions by preventing contact of the air/fuel mixture with the oil, crevices and deposits. The known methods of achieving this involve additional inlet ports or valves which are used solely for the induction of recirculated exhaust gas or allow air and exhaust gas to mix before or while passing through the inlet valve. Engines of both these types are disclosed in U.S. Pat. No. 4,193,382.
The disadvantage of such systems is that the additional opening into the combustion chamber adds cost and complexity and limits the space available for the other features of the combustion chamber such as intake and exhaust valves and the spark plug. The introduction of exhaust gas into the air stream before the inlet valve leads to substantial mixing and thus ineffective stratification and thus results in the disadvantages referred to above.
U.S. Pat. No. 4,393,853 discloses a four cylinder engine in which each cylinder has a single inlet port and a single exhaust port formed in the cylinder head and a further tangentially directed exhaust outlet/EGR inlet formed in the cylinder wall and positioned to be exposed by the piston at 60.degree. after the top dead center position. The exhaust outlet/EGR inlets of the four cylinders are connected together in two pairs. The inlet port is of helical type whereby on the induction stroke the inlet air is introduced into the cylinder to form a body of air which swirls around the cylinder axis. At low load, when the exhaust outlet/EGR inlet is exposed by the piston recirculated exhaust gas is introduced tangentially into the cylinder from the cylinder to which it is connected so that it swirls in the same direction as the air. The exhaust gas remains substantially outside the air and the contents of the cylinder are therefore nominally radially stratified. On the exhaust stroke the exhaust gases are all expelled in the usual manner through the exhaust valve in the cylinder head except for that small proportion which is required by the associated cylinder and this is discharged through the exhaust outlet/EGR inlet opening.
Quite apart from the fact that it has been found that radial stratification is unsatisfactory due to the fact that the two rotating bodies of gas tend to become considerably mixed, it is inconvenient to have to position an opening in the cylinder wall. Thus the presence of such an opening leads to high wear of the piston rings and to distortion of the cylinder wall which results in high oil consumption. Furthermore, the piston tends to scrape lubricating oil into the opening and the exhaust gas thus contains a relatively high proportion of unburnt or partially burnt oil.
In modern engines with four valves per cylinder it is common practice to induce air into the cylinder such that a tumbling motion occurs, that is to say rotation about an axis which is at right angles to the cylinder axis.
A known method of providing a stratified charge with a tumble inducing combustion chamber is described in a paper issued by the Society of Automotive Engineers, namely SAE 920670. This discloses a combustion chamber with two intake valves which have ports designed to provide tumbling air motion. A fuel injector is positioned such that a fuel and air mixture is induced through one of the intake valves but air alone is induced through the other intake valve. This produces horizontal stratification in the cylinder such that a rich fuel/air mixture exists on one side of the cylinder and a very weak mixture, or pure air, exists at the other side. The spark plug is positioned within the rich mixture zone so that the fuel can be easily ignited.
The disadvantage of this system is that which applies to other lean burn technologies. That is the presence of excess oxygen in the exhaust gas prevents the use of a reducing catalyst.